Add test to ensure that /api/doc/ responds, and fix path to doc from version file.
There is very odd classloading logic in spark for static files (as of 2.5). If you have a resource directory (like we do) it barfs if you try to capture that directory name as a "file-like" path with no trailing slash. This feels like a bug in Spark, but one I'm nonetheless not going to work around quickly. The link from the application version page now works, and we have a basic test to ensure that documentation is indeed being hosted correctly.